Multiple carrier modulation systems are known in communication applications. Multiple frequency carriers within an assigned channel, i.e. frequency block, are used for transmitting data. Each of the frequency carriers is modulated with a particular set of data to be transmitted, and power is assigned to each of those frequency carriers.
As shown in prior-art FIG. 1, all of the frequency carriers, carriers 115, of an assigned data channel, identified by full bandwidth 110, can be used to transmit data concurrently. Full bandwidth 110 includes a range of frequencies from a minimum frequency F1 to a maximum frequency F2. As each carrier of carriers 115 is associated with a power, a power spectral density (PSD), such as full bandwidth PSD 120, with a root-mean-square (rms) power level of P1, can be associated with a transmission of carriers 115. Full bandwidth PSD 120 represents a PSD associated with a transmission using a majority of the frequency carriers associated with full bandwidth 110 concurrently. However, not all of the frequency carriers of carriers 115, within full bandwidth 110, can necessarily be received well by a receiving system.
A receiving system can have problems identifying data from some of the carriers of carriers 115. Noise within the full bandwidth 110 can be too large for carriers to be distinguished. Noise floor 130 identifies an average level of noise within the channel having an rms power level PN. If the power associated with the carriers is not significantly greater than PN, the data associated with the carriers may not be properly identified. Furthermore, interference from other carriers or other signals can make data reception difficult. Noise and interference affect the probability that data will be reliably received across a particular channel. To improve channel reliability, the power associated with carriers 115 should be greater than the noise or interfering signals within the channel, or full bandwidth 110.
A prior art method of making the carriers distinguishable over noise or interference is to increase the power of the carriers well above the power of the noise or interference. As shown in prior art FIG. 2, a power associated with the carriers 215 within the channel, identified by full bandwidth 110, has been increased. A new PSD, modified PSD 220, is shown to identify a PSD, with a maximum power level of P2, associated with carriers 215 at the increased power levels. The increase in power makes the carriers 215 distinguishable over the noise floor 130. A nominal PSD 225 is used to identify a PSD, at an average power level of P1, of the carriers 215 at a normal power level, such as full bandwidth PSD 120 (prior art FIG. 1). A PSD mask associated with nominal PSD 225 is often used to identify average PSD levels that are not to be exceeded, according to particular communications specifications. To avoid transmissions of the data channel, which may interfere with transmissions of nearby data channels, the PSD of the data channel is kept within the nominal PSD 225. However, by increasing the power of the carriers to be well above the noise and interference, modified PSD 220 well exceeds the nominal PSD 225, possibly creating interference for other data channels. From the above discussion, it should be apparent that an improved method of increasing channel reliability, while not compromising a quality of adjacent channels, is needed.